Such coils are connected either directly between the line and neutral of a grid which is generally at high voltage, or else across the terminals of compensation windings belonging to transformers or auto-transformers. In all cases, provided they are connected to their rated voltage, the reactor coils have the unusual property of consuming their rated power regardless of load. Thus at constant voltage they are the seat of constant losses. It is therefore highly desireable to minimise the losses as much as possible. Two of the best known methods of reducing losses are: (1) to increase the mass of copper in order to reduce current density, thereby reducing losses in the windings; and (2) to increase the mass of the magnetic laminations in order to reduce flux density, thereby reducing losses in the magnetic circuit.
Unfortunately, these solutions not only increase the mass of the copper and/or the laminations, but they also lead to increases in the amount of metalwork and dielectric because of the greater overall dimensions. Increasing the mass of the laminations also has the effect of increasing noise, vibration and localised "hot spots" of maximum heating.
There is also the problem that a large shunt winding may produce several times as much magnetic induction in the vicinity of the winding supports, the magnetic circuit, and the cladding (or master) laminations thereof, than would a transformer in its corresponding portions. In addition to the increased losses this effect leads to severe hot spots.
British Pat. No. 983,481 describes a shunt reactor coil. With reference to FIGS. 1 and 2 in said patent specification, the coil comprises yoke members 2 whose width is equal to the diameter of the winding 1. The compression forces due to the electromagnetic forces are supported by a central column 3 of insulating material, but since the yoke members 2 extend beyond the diameter of the column 3 there is a danger that they will bear directly on the winding. To ensure that the forces are supported only by the central column 3, beams 9 are welded to the yoke members 2 to make them rigid enough to keep the forces off the winding. A modified construction is shown in FIG. 3 of the patent specification and is described at page 3 lines 33 to 58. In the modified construction the yoke members are no wider than the diameter of the support column 3, and auxiliary C-shaped laminated yoke members 10 are placed astride the yoke members so that the forces applied to the auxiliary yoke members are transmitted first to the yoke members 2 and thence to the support column 3.
Although such an arrangement effectively gets rid of the hot spots, it still suffers from drawbacks. The tips of the C-shaped laminated yoke members hang over the ends of the winding where they interfere with the requirement to leave an isolating clearance for connection leads to the winding. Further, in the case of large size inductance windings, the C-shaped laminations must either be built up from three pieces, or else a large size lamination cutting machine must be specially built to cut out C-shaped laminations directly from sheet material.
Either way, the C-shaped laminations are complicated and expensive to produce.
Preferred embodiments of the invention mitigate these drawbacks.